1. Field of the Invention
This invention relates to Al—Zn—Mg—Cu alloys with improved static mechanical characteristics—damage tolerance ratio, and having a Zn content preferably greater than 8.3%, as well as structural elements for aeronautical construction incorporating refined and/or partially finished products manufactured from these alloys.
2. Description of Related Art
Al—Zn—Mg—Cu alloys (belonging to the family of 7xxx alloys) are currently in use in aeronautical construction, and particularly in the construction of civilian aircraft wings. For the exterior of the wings, a skin (wingskin) of plate made in 7150, 7055, 7449 alloys is often used, and optionally stiffeners (also called stringers) made from profiles in 7150, 7055, or 7449 alloys. These designations of alloys, as is well known in the art, correspond to those of The Aluminum Association.
Some of these alloys have been known for decades, such as for example 7075 and 7175 (zinc content between 5.1 and 6.1% by weight), 7050 (zinc content between 5.7 and 6.7%), 7150 (zinc content between 5.9 and 6.9%) and 7049 (zinc content between 7.2 and 8.2%). Such alloys have a high tensile yield strength, as well as good fracture toughness and good resistance to stress corrosion and to exfoliation corrosion. More recently, it has appeared that for certain applications, alloys with a higher zinc content can have certain advantages, such as having an increased tensile yield strength. 7349 and 7449 alloys have a zinc content between 7.5 and 8.7%. Wrought alloys higher in zinc have been described in the literature, are not typically used in aeronautical construction.
U.S. Pat. No. 5,560,789 (Pechiney) discloses an alloy composed of Zn 10.7%, Mg 2.84%, and Cu 0.92% which is transformed by extrusion. These alloys are not designed specifically to have an optimized static mechanical characteristic to toughness ratio.
U.S. Pat. No. 5,221,377 (Aluminum Company of America) discloses several Al—Zn—Mg—Cu alloys with a zinc content of up to 11.4%. These alloys are deficient in certain respects in terms of properties, as will be explained hereinbelow.
Moreover, it has been proposed to utilize high zinc containing Al—Zn—Mg—Cu alloys to manufacture hollow bodies intended to resist increased pressures, such as for example, compressed gas cylinders. European Patent Application EP 020 282 A1 (Société Métallurgique de Gerzat) discloses alloys with a zinc content of between 7.6% and 9.5%. European Patent Application EP 081 441 A1 (Société Métallurgique de Gerzat) discloses a process for obtaining such cylinders. European Patent Application EP 257 1 67 A1 (Société Métallurgique de Gerzat) states that no known Al—Zn—Mg—Cu alloys can safely and reproducibly satisfy the strict technical demands imposed by this specific application for gas cylinders. EP 257 1 67 A1 proposes moving towards a lower zinc content, namely between 6.25% and 8.0%. The teaching of these patents is specific to problems relating to compressed gas cylinders, particularly concerning maximizing the bursting pressure of these cylinders, and thus cannot be transferred to other wrought products.
Generally in Al—Zn—Mg—Cu alloys, not only is a high zinc content desirable, but Mg and Cu are also generally included in order to obtain good static mechanical characteristics (ultimate tensile strength (Rm or UTS) and tensile yield strength (Rp0.2 or TYS).). This is only possible if these elements (Zn, Mg, Cu) can be put into solid solution. It is also well known (see, for example U.S. Pat. No. 5,221,377) that when the zinc content is increased in a 7xxx alloy beyond around 7 to 8%, then problems associated with insufficient resistance to exfoliation corrosion and stress corrosion will arise. More generally, it is known that the most charged Al—Zn—Mg—Cu alloys are likely to pose corrosion problems. These problems are generally resolved by employing specific thermal or thermomechanical treatments, especially by pushing the aging treatment beyond the peak, for example during a type T7 temper or treatment. But such treatments can then cause a corresponding drop in the static mechanical characteristics. In other words, in order to obtain a given minimum level of resistance to corrosion for an Al—Zn—Mg—Cu alloy, one must find a compromise between static mechanical characteristics (TYS Rp0.2, UTS Rm, and elongation at fracture A) and damage tolerance characteristics (fracture toughness, crack propagation rate etc.). According to the desired minimal level of resistance to corrosion sought to be obtained, either (i) a temper close to peak strength is utilized (T6 tempers), which generally offers an acceptable toughness to TYS ratio favouring static mechanical characteristics, or (ii) annealing is pushed beyond the peak strength (T7 tempers), by seeking a compromise favouring fracture toughness. These metallurgic states are defined in standard EN 515.